Total sky imaging systems are powerful tools for various scientific and industrial applications such as high resolution cloud tracking and short-term (less than one hour) solar power forecasting. The contrast required by sky imaging (˜1:1,000,000), however, far exceeds the capability of currently available digital imaging sensors (typically 1:1,000 or less dynamic range). A cloudy twilight sky (no direct sun) has a radiance of ˜0.1 milliwatts per square centimeter steradian (mW/cm2 sr) while direct sun illumination from a clear sky at noon has radiance ˜1×105 mW/cm2 sr. Imaging twilight sky demands a camera system without significant light attenuation.
On the other hand, the high intensity direct sun light without significant attenuation impinging on imaging sensors causes pixel saturation and blooming, i.e., accumulation of photoelectrons exceeds the designed linear-response and charge-holding capabilities of sensor pixels (saturation), which causes the excessive electrons to overflow to its neighboring pixels (blooming). Blooming causes artifacts in sensor output which include large image area around sun being smeared out, streak patterns on image, and erroneous pixel values. In extreme cases, even complete failure of a sensor can result from blooming due to breakdown of the preamplication stage.
To avoid sensor saturation and blooming, the commercial sky cameras available today typically use automated sun-tracking mechanical blockers to prevent direct sun light from entering the camera. The usage of mechanical sun blockers increases the cost of sky cameras and decreases their reliability in harsh field environments. Moreover, information on the sun and the part of the sky adjacent to the sun (typically around a 10 degree field of view), which is most important for solar power forecast, is lost because of the blocking. Moreover, because the sun is blocked, such sky camera systems can only image the sky around the sun and cannot measure the solar radiation. Thus several additional radiometric instruments such as pyronameters, spectroradiometers, and pyrheliometers (require precise sun tracking mechanism) are typically deployed along with sky cameras to measure the direct and diffusive irradiance from the sun. This significantly increases the overall system complexity and cost.
Therefore, improved total sky imaging systems that overcome the above-described drawbacks of conventional technologies would be desirable.